Thoughts as sub-vocal speech are more or less volitional as speaking is a kind of deliberate doing. Such thoughts can be at roughly the same speed as pronounced speech but can also occur much faster so that it takes on the character of another voice speaking inside your head (but with your tone and with a residual sense of accompanying volition). A tune that is stuck in your head is both volitional and compulsive and indicates the way that the deliberate and automatic are not clearly distinguished. When all thoughts of these kinds are silenced, as when in a state of more or less focused attention, there remains a welter of very fast thoughts which are not vocalised in any sense and are entirely automatic. They correspond to the flame-like waverings of attention, and arise as a something like the chromatic aberration at the edges in a photographic image. These are whole worlds compressed into many fine layers, like the entrances to dreams. The wavering of attention is an effect of will since in order to keep attention focused you need to present it with an object and hence you need to straddle it with a certain overview. This entire complex, the will, the overview, the attention and the upwelling of bright parasitic thoughts is what is known as the 'default mode'. Attempting to suppress it only feeds it, you can only inquire as to where it could be that such activity arises so seeming naturally.